Re: [PATCH v4] Introduce v3 namespaced file capabilities

[Stefan Berger]

On 06/20/2017 08:19 AM, Stefan Berger wrote:
> On 06/20/2017 01:42 AM, Amir Goldstein wrote:
> > On Tue, Jun 20, 2017 at 12:34 AM, Eric W. Biederman
> > <ebiederm@xxxxxxxxxxxx> wrote:
> > > "Serge E. Hallyn" <serge@xxxxxxxxxx> writes:
> > >
> > > > Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> > > > > On 06/14/2017 11:05 PM, Serge E. Hallyn wrote:
> > > > > > On Wed, Jun 14, 2017 at 08:27:40AM -0400, Stefan Berger wrote:
> > > > > > > On 06/13/2017 07:55 PM, Serge E. Hallyn wrote:
> > > > > > > > Quoting Stefan Berger (stefanb@xxxxxxxxxxxxxxxxxx):
> > > > > > > > >   If all extended
> > > > > > > > > attributes were to support this model, maybe the 'uid' could be
> > > > > > > > > associated with the 'name' of the xattr rather than its 
> > > > > > > > > 'value' (not
> > > > > > > > > sure whether that's possible).
> > > > > > > > Right, I missed that in your original email when I saw it this 
> > > > > > > > morning.
> > > > > > > > It's not what my patch does, but it's an interesting idea.  Do 
> > > > > > > > you have
> > > > > > > > a patch to that effect?  We might even be able to generalize 
> > > > > > > > that to
> > > > > > > No, I don't have a patch. It may not be possible to implement it.
> > > > > > > The xattr_handler's  take the name of the xattr as input to get().
> > > > > > That may be ok though.  Assume the host created a container with
> > > > > > 100000 as the uid for root, which created a container with 130000 as
> > > > > > uid for root.  If root in the nested container tries to read the
> > > > > > xattr, the kernel can check for security.foo[130000] first, then
> > > > > > security.foo[100000], then security.foo.  Or, it can do a listxattr
> > > > > > and look for those.  Am I overlooking one?
> > > > > >
> > > > > > > So one could try to encode the mapped uid in the name. However, 
> > > > > > > that
> > > > > > I thought that's exactly what you were suggesting in your original
> > > > > > email?  "security.capability[uid=2000]"
> > > > > >
> > > > > > > could lead to problems with stale xattrs in a shared filesystem 
> > > > > > > over
> > > > > > > time unless one could limit the number of xattrs with the same
> > > > > > > prefix, e.g., security.capability*. So I doubt that it would work.
> > > > > > Hm.  Yeah.  But really how many setups are there like that?  I.e. if
> > > > > > you launch a regular docker or lxd container, the image doesn't do a
> > > > > > bind mount of a shared image, it layers something above it or does a
> > > > > > copy.  What setups do you know of where multiple containers in 
> > > > > > different
> > > > > > user namespaces mount the same filesystem shared and writeable?
> > > > > I think I have something now that accomodates userns access to
> > > > > security.capability:
> > > > >
> > > > > https://github.com/stefanberger/linux/commits/xattr_for_userns
> > > > Thanks!
> > > >
> > > > > Encoding of uid is in the attribute name now as follows:
> > > > > security.foo@uid=<uid>
> > > > >
> > > > > 1) The 'plain' security.capability is only r/w accessible from the
> > > > > host (init_user_ns).
> > > > > 2) When userns reads/writes 'security.capability' it will read/write
> > > > > security.capability@uid=<uid> instead, with uid being the uid of
> > > > > root , e.g. 1000.
> > > > > 3) When listing xattrs for userns the host's security.capability is
> > > > > filtered out to avoid read failures iof 'security.capability' if
> > > > > security.capability@uid=<uid> is read but not there. (see 1) and 2))
> > > > > 4) security.capability* may all be read from anywhere
> > > > > 5) security.capability@uid=<uid> may be read or written directly
> > > > > from a userns if <uid> matches the uid of root (current_uid())
> > > > This looks very close to what we want.  One exception - we do want
> > > > to support root in a user namespace being able to write
> > > > security.capability@uid=<x> where <x> is a valid uid mapped in its
> > > > namespace.  In that case the name should be rewritten to be
> > > > security.capability@uid=<y> where y is the unmapped kuid.val.
> > > >
> > > > Eric,
> > > >
> > > > so far my patch hasn't yet hit Linus' tree.  Given that, would you
> > > > mind taking a look and seeing what you think of this approach?  If
> > > > we may decide to go this route, we probably should stop my patch
> > > > from hitting Linus' tree before we have to continue supporting it.
> > > Agreed.  I will take a look.  I also want to see how all of this works
> > > in the context of stackable filesystems.  As that is the one case that
> > > looked like it could be a problem case in your current patchset.
> > Apropos stackable filesystems [cc some overlayfs folks], is there any
> > way that parts of this work could be generalized towards ns aware
> > trusted@uid.* xattr?
>
> I am at least removing all string comparison with xattr names from the 
> core code and move the enabled xattr names into a list. For the 
> security.* extended attribute names we would enumerated the enabled 
> ones in that list, only security.capability for now. I am not sure how 
> the trusted.* space works.

I extended 'the infrastructure' now to support prefix matching for 
trusted.* and probably others as well. It's fairly easy to do that but 
would not write the code like that for exact string matching to support 
security.capability. The patch lets me write trusted.foo@uid=100 from 
within the userns if uid=100 exists, rejects it otherwise. It may be 
written out as trusted.foo@uid=1100 for root mapping to uid 1000. I can 
list this entry on the host. For some reason trusted.* is not listed at 
all inside the userns. So something else needs to be enabled as well. 
For now it looks like this:


https://github.com/stefanberger/linux/commit/8ae131e731c9e1def92a2100697632ea35e007d0

Regards,
    Stefan